Conventional abrasive sheets are produced by first forming a uniform and flat layer of a slurry, which may be a mixture of abrasive particles and a binder, on the surface of a plastic base film material by using a roller or a doctor blade and then drying this slurry to provide an abrading layer (as disclosed, for example, in Japanese Patent Publication No. 53-44714). On the surface of the slurry on such a prior art abrasive sheet, however, many protrusions and indentations are formed during the drying process of the slurry, although the slurry surface is once made flat and smooth by using a roller or a doctor blade prior to the drying process. Thus, such an abrading sheet could not be suitably used for a precise finishing process such as finish abrading because protruding parts of its uneven slurry surface would abrade a workpiece excessively.
The manner in which such undesirable unevenness is formed on the surface of the abrading layer is explained next with reference to FIGS. 6A and 6B. FIG. 6A is a sectional view of a prior art abrasive sheet when a plastic sheet (serving as its base sheet) 6 has just been coated with a slurry 3 comprising abrasive particles 1 and 1' and a binder 2 and its surface has been flattened, as explained above, by means of a roller or a doctor blade. As the slurry 3 is dried, however, the solvent is evaporated from the binder 2 and its heat of vaporization causes the temperature of the binder 2 to drop. If there is a relatively large abrasive particle (as indicated by 1' in FIGS. 6A and 6B) near the surface, evaporation of the solvent does not take place therearound and the temperature of the binder 2 does not drop significantly in its neighborhood. In other words, the temperature of the binder 2 becomes higher near large abrasive particles 1' than elsewhere, and this means that the solvent of the binder 2 evaporates more actively and hence dries up the slurry 3 more quickly near large abrasive particles 1'. As a result, the surface tension of the binder 2 becomes greater near large abrasive particles 1', causing the surface layer of the binder 2 to shift, as indicated by arrows 13 in FIG. 6B.
In regions where the concentration of the binder 2 is relatively high, the concentration of its solvent is also relatively high and hence there is more fluidity. Thus, those relatively large abrasive particles 1' are moved in the direction of the surface tension (or the arrows 13), dragging the slurry material therewith and forming protrusions 11 where they settle and indentations 12 between the protrusions 11, as shown in FIG. 6B.
There have been attempts to precisely control the average size of the abrasive particles in order to prevent the formation of such protrusions, but it was a difficult goal to attain. Even if the average particle size is controlled or the surface of the slurry layer is made flat before it is dried, the slurry surface becomes uneven after a drying process, resulting in an abrasive sheet with an uneven abrading surface having protrusions and indentations distributed in an irregular manner. If a workpiece is abraded by such an abrasive sheet, the protrusions on the abrading surface will cut deeply into its surface. In other words, a precise processing such as a finish abrading could not be carried out successfully with prior art abrasive sheets.